This invention relates generally to pressure measuring devices and more specifically to fiber-optic pressure transducers.
In the art of pressure measuring, various devices based on the vibrating wire technique have been used to measure pressure. In one of these devices a length of wire is mounted in tension between a pressure sensitive diaphragm and a fixed mount in a controlled environment, such as an evacuated housing. The housing is formed of nonmagnetic materials so that the wire may by plucked by the use of an electromagnetic actuator positioned outside of the housing. Pressure applied to the diaphragm causes it to deflect and alter the tension on the wire, thereby altering the resonant frequency of the wire when plucked. The vibrating frequency of the wire is sensed, usually by the same electromagnetic coil which energizes the wire, and the frequency vibration is determined and converted to a calibrated measurement of the pressure applied to the diaphragm.
In these systems, the wire is housed in an evacuated housing and the housing materials are carefully selected in an effort to minimize errors introduced by changes in temperature. Also, it is difficult to mount the wire in the housing in a manner that eliminates creep in the wire which introduces errors into the measurement technique. These disadvantages limit the environments in which this type of pressure transducer may be used. In addition, these instruments are susceptible to stray electromagnetic or electrostatic interference, which further reduces there applications.
In order to overcome some of the disadvantages of vibrating wire type pressure transducers, optical-fiber pressure transducers have been used in which an optical fiber is mounted on one side of the pressure sensitive diaphragm similar to the arrangement of a strain gauge and the attenuation of light passing through the fiber is measured as an indication of the sensed pressure. As pressure is applied to the diaphragm in this arrangement, the fiber bends with the diaphragm causing the light transmitted through the fiber to be attenuated by an amount which increases with the curvature of fiber bending. Although this instrument is not susceptible to electromagnetic or electrostatic interference, it has not been found to be a reliable pressure measuring instrument due to excessive ambient temperature sensitivity (5 to 10% FS drift/50.degree. F.).
Thus, there is a need for an accurate pressure transducer which is operable in varying environmental conditions and which may be used in areas where the instrument is exposed to electromagnetic or electrostatic interference.